In the shape measurement for a to-be-measured object such as a semiconductor wafer, shape determining devices of non-contact type employing an interferometer are spread widely. In such a device, on the basis of the intensity of an interference light beam where a reference light beam and an object light beam having the same wavelength are superposed, the surface shape of a to-be-measured object, that is, distribution of the surface height (or distribution of the position of the surface), is acquired. Here, a light beam obtained when one of two branched light beams is reflected in the surface of the to-be-measured object is an object light beam, and a light beam obtained when the other light beam is, for example, reflected in a reference surface serving as a reference and is not projected onto the to-be-measured object is a reference light beam.
More specifically, in the surface shape measurement for a to-be-measured object by using an interferometer, an interferometer arranged opposite to the surface of the to-be-measured object detects the intensity of an interference light beam obtained by interference between an object light beam reflected at many measurement sites on the surface of the to-be-measured object and a reference light beam. At that time, at each measurement site, the intensities of plural kinds of interference light beams are detected by a method that the optical path length of the reference light beam is changed or the like so that the difference in the phases of the object light beam and the reference light beam is shifted by a predetermined amount. Then, for each measurement site, the phase difference between the object light beam and the reference light beam is calculated from the intensities of the acquired plural kinds of interference light beams at the measurement site, and then phase connection processing is performed on the basis of the data of phase differences at the plurality of measurement sites. The phase data at each measurement site acquired by this phase connection processing can be converted into a dimension value for the surface height on the basis of the wavelength of the object light beam. Thus, the distribution information of the phase data acquired by the phase connection processing is equivalent to distribution information of the surface height of the to-be-measured object, that is, shape information. Here, the phase connection processing is referred to as unwrapping processing.
By virtue of this, the surface shape of the to-be-measured object can be measured in a non-contact manner. Thus, in comparison with a case of measurement employing a shape measuring instrument of sensing pin type, the surface shape can be measured without causing scratches or the like in the to-be-measured object surface.
Patent reference 1 describes the details of phase connection processing. Patent reference 1 describes a technique of measuring a characteristics change in a fluid accommodated in a cell by detecting a change in the phase of the interference light beam obtained by superposition of an object light beam having passed through the cell and another light beam for reference. At that time, the phase data is sampled with a predetermined period. Further, phase connection processing is performed in which the phase in the phase data at a particular time is shifted by an integral multiple of 2π such that for the phase data sampled at the particular time point, the phase difference falls within the range from −π to +π with reference to the phase data at the preceding time point.
Similarly, in the phase connection processing in the shape measurement, correction processing is performed on one phase of the two phase data pieces acquired at two adjacent measurement points. In this processing, correction is performed on the phase at one of the two adjacent measurement points by an integral multiple of 2π such that the phase difference falls within the range from −π to +π with reference to the phase at the other point. The phase connection processing performed as described here depends on a premise that the difference between the surface height values at two adjacent measurement points does not exceed ¼ of the wavelength of the object light beam.
Patent reference 2 describes a two-dimensional information acquiring device in which three interference light beams are acquired by the following method.
That is, in the device described in Patent reference 2, a parallel light beam obtained by expanding a laser light beam is projected onto a reference surface and a to-be-measured surface so that a sensing light beam (a non-interference light beam) is acquired that contains a reference light beam and an object light beam as mutually orthogonal polarization components. Further, the sensing light beam is branched into three. Then, from the three branched light beams, the three polarizing plates extract polarization components having mutually different polarization angles, so that three interference light beams are acquired in each of which the phase difference between the components of the reference light beam and the object light beam is shifted by 90°. As such, when the phase shift onto the reference light beam and the object light beam is performed optically by using several polarizing plates whose polarization components to be extracted are different from each other, the plurality of interference light beams having undergone the phase shift are acquired simultaneously. Then, from the intensities of the plurality of interference light beams, the phase difference can be calculated between the reference light beam and the object light beam. Then, distribution of the surface height of the to-be-measured object can be calculated from the distribution of the phase difference.
As such, in the technique described in Patent reference 2, phase shift is performed optically by using polarizing plates in shape measurement according to a phase shift method employing a homodyne interferometer. According to this technique, high-speed measurement can be achieved in comparison with a case of shape measurement according to a general phase shift method in which the position of a reference surface is mechanically moved sequentially so that a changed is generated in the phase difference between the reference light beam and the object light beam.
Patent reference 3 describes a device in which each of two heterodyne interferometers arranged opposite to the front and the back surfaces of a to-be-measured object detect beat signals of interference light beams where the relation between the reference light beam and the object light beam is reversed at a measurement site on the front and the back surfaces of the to-be-measured object, and thereby measure the thickness of the to-be-measured object on the basis of the phase difference between the front and the back beat signals.